Nature Notes: Bumble Bees, Blooms, & Bachelors
If you’re looking to get an up-close look at bumble bees, late summer and early fall is the time to do it. You can often find “sleeping” bumble bees lethargically lounging on late-season blooms, especially early in the morning. Bees are ectothermic, meaning they rely on the environment for heat. When it’s in the low 50s or colder, bumble bees’ muscles can’t function well enough to fly long distances or sometimes at all! Cool fall mornings offer the perfect opportunity to observe them closely, without fear of repercussions. But what are these bees doing out during this time of year? Let’s take a step back and look at the annual cycle of a bumble bee colony.
Bumble Bees of Minnesota & Wisconsin
All bumble bees share the genus Bombus and are one of the only types of native North American bees that are truly eusocial - a single female producing offspring and roles among workers - much like the non-native European honey bees. Historical records in Minnesota and Wisconsin show 24 and 20 different species of bumble bees, respectively, but populations are declining due to habitat loss, pesticides, pathogens, and climate change. Both states show a 25% decrease in bumble bee species in recent years. Bumble bees are uniquely suited to colder climates due to their densely hairy bodies, making them important pollinators in the very early spring and late fall, when many other pollinators are not active. They also do something called “buzz pollination” or “sonication,” where they grab a flower and use vibrations from their flight muscles (they can beat their wings 130x/second!) to shake loose pollen that would otherwise be difficult to dislodge. It is estimated that about 9% of plants worldwide rely on sonication, including tomatoes!
The annual colony cycle starts in the spring. Queen bees that spent winter hibernating underground will emerge as the sole survivors. They are known as “foundresses” and are tasked with starting a new colony. They find a suitable pre-existing cavity for their nests, which provides insulation and protection from the elements. While they prefer sites underground, like old rodent burrows, they’ll also nest above ground in places like tree hollows, abandoned bird nests, rock walls, grass tussocks, or in man-made structures like birdhouses or the cotton batting of old furniture. Queens seem to be pretty adept at picking locations seldom found by humans. You can read more about them and see some examples in this article from Xerces.
Busy Bumble Bees
When a suitable spot is found, the foundress gets to work. As a one-lady show, she is responsible for building the nest, laying the eggs, foraging for food (for herself and her brood), and incubating the eggs in the cold temperatures of early spring! She creates waxy cups (brood cells) in the nest and places pollen, nectar, and an egg in each. Her time is split equally between foraging and tending to the nest/incubating the eggs. When the eggs hatch, the bee larvae feed on the flower resources in the brood cell before pupating and emerging as sterile adult females. This whole process takes about 4-5 weeks. These new worker bees will now help the queen build brood cells, forage, and tend to the next brood. Once a queen has enough workers, she will no longer leave the nest and put all her energy into egg production. Depending on the species, a bumble bee colony may have between 50 and 500 bees in it.
Scientists have found that larger bumble bees tend to have foraging roles outside the nest while smaller bumble bees tend to have roles inside the nest including caring for developing bees, nest construction, climate control, and colony defense. Unlike honey bees, which need to forage enough nectar and pollen to create honey for the hive to live off of during the winter, bumble bees only make a small amount of honey, enough to last a few days during inclement weather. Since most of the colony will die off in the fall, with the few remaining bees in hibernation, there is no need to produce a massive food store for winter. Instead, the goal of the hive is to have enough resources to successfully begin the next generation of bees.
Bumble Bee Bachelors & Blooms
In the late summer, queens will switch from laying eggs that produce sterile female worker bees to laying eggs that produce gynes (fertile female bees that will mate and become new queen bees) and drones (male bees). Gynes and drones will leave the nest in order to mate with bees from other colonies.
The nests that were able to forage more resources (often larger nests) are able to support the production of more gynes and drones, therefore increasing the odds the queen bee’s genetics will be passed on in new colonies in the following year. The gynes head out into the world to find a mate but may return to the nest to sleep at night and share resources with their sisters. When drones leave the nest, they never return. Instead, they live the bachelor life - foraging during the day, leaving scent for females to find them, and sleeping underneath flowers at night.
Scientists have found that temperatures underneath a flower, near the nectar source, and especially inside of a flower can be significantly warmer than surrounding areas. The inside of a squash blossom can be up to 18 degrees warmer than the ambient air! This gives them a huge advantage by limiting time spent in an immobile state. While most bumble bees sleeping near flower blooms are drones, occasionally you may find a female worker or gyne who got caught foraging during a rapid temperature decrease and was unable to make it back to the nest. How can you tell a male from a female? Check the legs! Females have a wide flattish section on their back legs known as a corbicula, or pollen basket, that is used to carry pollen back to the nest which males lack.
L: Female bee with empty pollen basket. Center: Female bee with full pollen basket. R: Male: Skinnier back legs, no pollen basket. 📷Nora W.
As temperatures continue to drop, the old queen or founder will begin to slow egg production and eventually die. The female workers will also begin to die out, though some may remain until the first hard frost. Males can be seen sleeping in or under flowers, often wet from dew and unable to move in the cold morning temperatures. Some will warm back up and be able to fly later in the day, and some will die, falling to the ground underneath the flowers, perhaps consumed by hungry birds or other animals. The new queens eventually leave the nest. Before the first freeze, they’ll gorge on nectar and pollen to fatten up for winter, just like a bear does. Then, she’ll find a nice cozy hole in the ground to hibernate in for winter, before emerging in spring to start a new colony, beginning the cycle all over again.
L: Bumblebee on a flower covered in morning dew. R: Same bee later in the day, still not moving. It's on its back & legs are curled under, indicating it is deceased. A beautiful final resting place. 📷Erika O.
Beeeee a Help To Bumbles!
If you’d like to help out the bumble bees in your area, check out our blogs on participating in bee citizen science projects or preparing your pollinator-friendly yard for winter! Data from Minnesota’s Bee Atlas citizen science programs has produced a list of native flowers most often used by bumble bees - so you could consider adding these natives to your yard. Check around and inside your flowers this fall to see if you can find some sleeping bumble bees. Enjoy the wonders of nature!